Lath and plaster is a building process used to finish mainly interior dividingwalls and ceilings. It consists of narrow strips ofwood (laths) which are nailed horizontally across thewall studs or ceilingjoists and then coated inplaster. The technique derives from an earlier, more primitive process calledwattle and daub.[1]
Lath and plaster largely fell out of favour in theU.K. after the introduction ofplasterboard in the 1930s.[2] InCanada and theUnited States, wood lath and plaster remained in use until the process was replaced by transitional methods followed bydrywall (the North American term for plasterboard) in the mid-twentieth century.[citation needed]
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The wall or ceiling finishing process begins with wood or metallaths. These are narrow strips ofwood, extruded metal, or split boards, nailed horizontally across thewall studs or ceilingjoists. Each wall frame is covered in lath, tacked at the studs. Wood lath is typically about one inch (2.5 cm) wide by four feet (1.2 m) long by1⁄4 inch (6 mm) thick. Each horizontal course of lath is spaced about3⁄8 inch (9.5 mm) away from its neighboring courses. Metal lath is available in 27-inch (69 cm) by 8-foot (240 cm) sheets.[3]
InCanada and theUnited States the laths were generally sawn, but in theUnited Kingdom and its colonies,riven or split hardwood laths of random lengths and sizes were often used. Early American examples featured split beam construction, as did examples put up in rural areas of the U.S. and Canada well into the second half of the 19th century. Splitting the timber along its grain greatly improved the laths' strength and durability. As Americans and Canadians expanded west, saw mills were not always available to create neatly planed boards and the first crop of buildings in any new western or northern settlement would be put up with split beam lath. In some areas of the U.K. reed mat was also used as a lath.
Temporary lath guides are then placed vertically to the wall, usually at the studs.Lime orgypsumplaster is then applied, typically using a wooden board as the application tool. The applier drags the board upward over the wall, forcing the plaster into the gaps between the lath and leaving a layer on the front the depth of the temporary guides, typically about1⁄4 inch (6.4 mm). A helper feeds new plaster onto the board, as the plaster is applied in quantity. When the wall is fully covered, the vertical lath "guides" are removed, and their "slots" are filled in, leaving a fairly uniform undercoat.
In three coat plastering it is standard to apply a second layer in the same fashion, leaving about1⁄2 inch (13 mm) of rough, sandy plaster (called abrown coat orbrowning (UK)). A smooth, white finish coat goes on last. After the plaster is completely dry, the walls are ready to be painted. In this article's photo ("lath seen from the back...") the curls of plaster are calledkeys and are necessary to keep the plaster on the lath. Traditionallime based mortar/plaster often incorporateshorsehair which reinforces the plasterwork, thereby helping to prevent the keys from breaking away.
In addition to wood lath, various types ofmetal lath began to be used toward the end of the 19th century.[4] Metal lath is categorized according to weight, type of ribbing, and whether the lath isgalvanized or not. Metal lathing was spaced across a 13.5-inch (340 mm) center, attached by tie wires using lathers' nippers. Sometimes, the mesh was dimpled to be selffurring.
In use as early as 1900,rock lath (also known as "button board," "plaster board" or "gypsum-board lath"), is a type ofgypsum wall board (essentially an early form of drywall) with holes spaced regularly to provide a 'key' for wet plaster.[4] Rock lath was typically produced in sheets sized 2 by 4 feet (610 by 1,220 mm). The purpose of the four-foot length is so that the sheet of lath exactly spans three interstud voids (overlapping half a stud at each end of a four-stud sequence in standard construction), the studs themselves being spaced 16 inches (410 mm) apart on center (United Statesbuilding code standard measurements). By the late 1930s, rock lath was the primary method used in residential plastering.[4]
Lath and plaster methods have mostly been replaced with moderndrywall orplasterboard, which is faster and less expensive to install. Drywall possesses poor sound damping qualities and can be easily damaged by moisture. Traditional lime based plasters are resistant to moisture and provide excellent sound isolation.
One continued advantage of using traditional lath is for ornamental or unusual shapes. For instance, building a rounded wall would be difficult if drywall were used exclusively, as drywall is not flexible enough to allow tight radii. Wire mesh, often used for exteriorstucco, is also found in combination or replacement of lath and plaster which serves similar purpose.
Traditional lath and plaster (including rock and metal lath varieties) has superior sound-proofing qualities when used with lime or gypsum plaster, which is denser than modern drywall.[2]
In many historic buildings lath and plaster ceilings play a major role for the prevention of fire spread. They are critical to the protection of horizontal elements such as timber joisted floors, including the flooring on top, which in terms of fire performance is often in a poor condition due to the presence of gaps.[5]
